Increasing performance, price, and reliability demands for advanced machinery such as modern high-speed compressors, gas turbines, steam turbines, gearboxes, and the like, have resulted in complex design requirements for bearings. Of the variety of bearing designs employed to minimize rotating shaft bearing instabilities, the tilting pad concept has been the most successful. But conventional flood-lubricated, mechanically-pivoted tilting-pad journal bearings have several disadvantages, including relatively high power loss, mechanical complexity, pivot fretting, limited damping capacity, and indirect measurement of bearing loading.
An alternative tilting pad journal bearing was developed which uses the hydrodynamic pressure formed by the rotation of the journal in the wedge-shaped lubricant film as a source of hydrostatic pressure to support each pad. Such hydrostatically supported tilting pad journal bearings are described in U.S. Pat. No. 3,549,215 and U.S. Pat. No. 4,059,318, assigned during their term to the assignee of the present invention, and hereby incorporated by reference into this application. The aforementioned designs avoid high power losses inherent with flooded lubrication in favor of providing lubrication directed to the leading and trailing edges of each pad. Lacking mechanical pivots, fretting is also avoided and bearing damping is enhanced by means of the hydrostatic film supporting each pad.
Among the factors which must be taken into account when designing such bearings are mechanically and thermally induced shaft misalignment. Conventional bearings often rely on a spherical fit between the outside diameter of the stationary, carrier ring, or as it is commonly referred to, bearing shell, and the inside diameter of the machine's housing to promote alignment between the shaft and bearing surfaces. In spite of several patents improving the spherical fit, such a purely mechanical solution has not proven entirely satisfactory. Frictional resistance to the motion of the bearing shell within this spherical fit reduces the capability of the bearing to track the axial tilt of the shaft. This results in uneven axial bearing loading and elevated, possibly damaging, localized temperatures.
In an industrial bearing application, the direct load applied by the rotating element to the bearing is generally not measured due to cost and conceptual limitations. In some machines temperature sensors or vibration sensors are used to provide an indication of expected relative bearing loading. But, these indirect measurements may instead reflect factors other than load and therefor are an inadequate measure of bearing loading. It is anticipated that direct measurement of bearing loading during operation would allow potential alignment correction of the machinery and aid in the diagnosis of operational performance or fault conditions; particularly ripe for such analysis would be rigidly-coupled rotating elements having more than two bearings per span which, in terms of bearing loading, can be described as "statically in determinant."
Two major areas of concern not yet satisfactorily addressed by bearing designs to date are 1) direct measurement of bearing loading and 2) stable bearing operation under varying conditions of axial tilt of the rotating element.